Conventionally, an optical disk has widely been used as a recording medium recording audio, video, document data, or other form of information. In addition, various optical disk devices for reproducing or recording information from or in optical disks have been developed. In the optical dick device, an optical pickup unit used for signal input or signal output for an optical disk constitutes a main component of the optical disk device.
FIG. 10 is a side view that schematically illustrates a structure of an ordinary optical pickup unit. An optical pickup unit 100 includes, roughly, a semiconductor laser 101, which is a light-source, a lens optical system 102, which condenses an emitted laser beam of the semiconductor laser 101 onto an optical disk 200, and a photo-receiving amplifier element 103, which receives reflected light from the optical disk 200.
The lens optical system 102 includes a prism (or a half mirror) 104, which is disposed in an optical path between the semiconductor laser 101 and the optical disk 200, and a condensing lens 105. The photo-receiving amplifier element 103 receives a laser beam emitted from the semiconductor laser 101 and reflected by the optical disk 200 and the prism 104.
The photo-receiving amplifier element 103 includes photo-receiving sections (photo diodes) each of which is divided into a plurality of photo-receiving regions, as shown in FIG. 11. In the configuration example in FIG. 11, focusing and data signal read out are carried out by a main photo-receiving section (regions A-D) 201 that is disposed in the middle and is divided into four regions. Tracking is carried out by sub photo-receiving sections (regions E-H) 201 that are disposed on the right side and left side of the main photo-receiving section. An amplifier circuit is connected to each of these photo-receiving sections.
FIG. 13 illustrates an equivalent circuit block in an amplifier circuit of the photo-receiving amplifier element 103. In FIG. 13, the photo-receiving section 301 corresponds to each of regions A-D in the main photo-receiving section 201, and regions E-H in the sub photo-receiving section 202. In the photo-receiving section 301, a received laser beam signal is converted into a current signal, and the current signal is outputted as current signal Isc. The current signal Isc is converted into a voltage and amplified by a first-stage amplifier A11 and resistor R11 making up a first-stage amplifier unit (including a feedback circuit). The output voltage is further amplified by an amplifier A12 and resistors R12, R13, and R14 of the next stage, and is outputted as a signal from an output terminal 302.
Examples of optical disk recording media include: CD, which employs an infrared laser and is mainly used for recording audio/text data; DVD, which employs an infrared laser and is mainly used for recording video data; and BD (Blue-ray Disk), which employs a blue laser to accommodate movement toward large data capacity as in digital broadcasting in the future.
The fundamental signal frequencies of the respective optical disks are 720 kHz for CD, 4.5 MHz for DVD, and 16.5 MHz for BD, substantiating the movement toward large data capacity (smaller recording pit). Further, with the movement toward faster response speed (reading and writing) in optical disk devices, faster response characteristics are also required for a photo-receiving amplifier element employed in an optical pickup unit. As an overview, the response frequency characteristics required for a photo-receiving amplifier element are 150 MHz for 16-times read out speed in DVD, and 200 MHz or greater for 4-times read out speed in BD.
In the photo-receiving amplifier element illustrated in FIG. 13, the frequency response characteristics are essentially restricted by the characteristics of the first-stage amplifier unit including the first stage amplifier A11 and the resistor R11. Thus, in order to obtain high-speed response characteristics in a photo-receiving amplifier element, it is necessary that the first-stage amplifier be speeded-up.
One method of increasing the speed of the first stage amplifier unit is to increase the speed of the respective elements of the first-stage amplifier unit by modifying the fabrication process of an integrated circuit, so as to increase the open-loop gain of the first-stage amplifier A11. Another method is to decrease the resistance of R11, which is a feedback resistor, and thereby increase the speed and band of the first-stage amplifier unit. Both of these methods are based upon the concept of the GB (gain-bandwidth) product, a relationship between gain and bandwidth.
The former method in which the respective elements are speeded up has a problem of requiring a long development period and huge costs for modifying the fabrication process of integrated circuits.
On the other hand, the latter method in which the resistance of the feedback resistor R11 is decreased has the following problem. Because the resistance of the feedback resistor R11 determines the sensitivity (photo-electronic signal conversion rate) of the photo-receiving amplifier element, the resistance of the feedback resistor R11 is set according to the optical design of the pickup and cannot be freely set in accordance with response frequency.
More specifically, the feedback resistor R11 in FIG. 13 converts photocurrent from the photo-receiving section 301 into a voltage signal, and predominantly decides the response characteristic of the photo-receiving amplifier element based on the product of gain and bandwidth. Because such photo-receiving amplifier element is usually formed on a single substrate by a semiconductor process, active elements such as transistors, and other elements such as resistors and capacitors are separately formed on an island (epitaxial layer) via junctions. As illustrated in FIG. 14, feedback resistors and other resistors are formed on the same island.
Integrated circuits inherently include a p-n junction for separating elements. As such, a parasitic resistance and/or parasitic capacitance is added to the respective elements. FIG. 15 shows an example of a structure of a resistor portion that is formed by a semiconductor process. In this example, an N-type epitaxial layer 402 is formed on a silicon substrate 401, and a polysilicon film 404 patterned into a resistor portion is formed thereon with a SiO2 film 403 in between. In addition, a SiO2 film 405 is formed on the polysilicon film 404. In this structure, capacitors C1 are formed between the polysilicon film 404 and the N-type epitaxial layer 402 with the SiO2 film 403 as a dielectric, and capacitors C2 are formed along the junction of the N-type epitaxial layer 402 and the silicon substrate 401. Here, the capacitors C1 and C2 serially provide a parasitic capacitance Cp against the resistor.
With the parasitic capacitance Cp added as a distributed constant circuit, the feedback resistor R11 is equivalently represented as shown in FIG. 16. Here, the feedback resistor R11 has a structure (characteristics) of an integrator due to the influence of the parasitic capacitance Cp, and the response frequency of the integrator is expressed as 1/(2π·R·Cp). In this manner, the feedback resistor portion in the first-stage amplifier unit is caused to have characteristics of an integrator (low pass). This limits the response frequency of the first-stage amplifier unit, making it difficult to increase the speed of the first-stage amplifier unit.